Record signatures origin in the signature processor of aggregator

[jpraynaud]

Why

We want to record the number of incoming signatures coming from the DMQ network. Once the DMQ is delivering enough signatures to reach the quorum, we will be able to sunset signature publication to the aggregator REST API.

What

Record the origin of the signatures from a SignatureConsumer with the existing metrics origin mechanism.

How

• Extend the SignatureConsumer trait to expose the origin metric of the consumer
• Increment the get_signature_registration_total_received_since_startup metric with the origin for every signature processed
• Modify usage of metric signature_registration_total_received_since_startup incremented in HTTP server when calling register-signatures with HTTP value